I'm pretty sure the Zu is a hybrid cable, both silver and copper. The Silver Dragon is a totally silver cable.
A good copper cable plays up the strengths of the HD-650 and improves on them, but these two change the sound more. The HD-650 is often a "dark" and "veiled" sounding headphone, so these cables help liven them up a bit.
I would describe the Zu cable as "fun" whereas the silver dragon is "analytical". That doesn't mean that one is better than the other, it's just what kind of sound you are looking for. The SD will bring the most clarity, detail, resolution, and speed, which is great for analyzing the music. It won't turn your headphone into an RS-1 or anywhere close, but it does get rid of the overly "dark/ veiled" sound. I would describle the SD as a "tight" and "clean" sound. The SD is great for tubes IMO because it brings out the most detail in the headphone and you still get all the mushy fun with the tubes.
The Zu is somewhere between the Silver Dragon and a copper cable like the Blue Dragon and Cardas. If your system is a little boring, then this would probably be the one to go for. It addresses many of the HD-650s weaknesses, while building up its strengths. It livens up the headphone and brings out more detail, but not quite as much as the SD. It does, however, bring forward the "recessed" midrange, and bring out the bass. The SD will have a tighter sounding bass, whereas the Zu will have a bigger, fuller sounding bass.
This is all IMO and different synergy / tastes may work better for others. I have not heard the Equinox or APS, so I cannot comment on either, but they do both get a lot of great feedback.